Although many genes and cDNAs encoding useful proteins have been isolated, a major problem facing scientists is the production of sufficient amounts of recombinant proteins for further study and for diagnostic or therapeutic applications. This is particularly so where commercial production of the protein is desired. In particular the production of hormones, cytokines, growth factors, receptors and their ligands and other proteins by recombinant means has become a major endeavour.
By way of example the cell surface molecule, CD46 (also known as membrane cofactor or MCP) has not been expressed in eukaryotic hosts as well as other cDNA constructs using a variety of expression vectors. Similar concerns have arisen about CD55 (DAF) and CD35 (CR1) and factor H which like CD46, are members of the family of proteins called Regulators of Complement Activation (RCA).
Regulators of Complement Activation control the amplification of the complement cascade. Each of the members of this family share structural characteristics, principally the .about.60-65 amino acid Short Consensus Repeat (SCR) modules, which are responsible for complement binding and regulatory functions. Genes encoding the family members are linked and map to chromosome 1q3.2.
The biology of these molecules is of interest in inflammation and homeostasis. However, there is currently a further application for them in the potential use of porcine organs or other organs for grafting to humans. Such xenogenic organs undergo antibody and complement mediated hyperacute rejection.
A limitation to studies of CD46 and other RCAs has been that on transfection with constructs encoding the recombinant proteins in either transient or stable systems very few cells have been transfected and those that are express low amounts of protein. Indeed, in the inventors' laboratory and others, sophisticated cell sorting, cloning and selection procedures have been necessary to isolate appropriate cells. This, in particular, has created problems with the analysis of recombinant CD46, its physiological role and value as a therapeutic agent.
In work leading up to the present invention the inventors have surprisingly found that alterations in a nucleic acid encoding CD46 led to high levels of CD46 production in a transfected eukarotic host. Protein production levels of up to 8 times that obtained from the wildtype gene for CD46 have been observed. These alterations comprised reducing the amount or proportion of adenine (A) and/or thymine (T) in an A and/or T rich region in the nucleic acid.